The way I remember it, locals versus globals and a heap of other ideas more or less came into
being along with the 'Structured' revolution. If you want a good (possibly best) introduction
to some of the pre-object ideas, you can't do much better than Structured Design by
by Ed Yourdon and Larry Constantine (Prentice Hall/Yourdon Press, 1978.) And for an honest and funny
appraisal see Ed Yourdon's self-review.

More current thinking (i.e. oo) would say that variable scoping is tied to the idea of information
hiding. In a kind of paranoid fashion, the goal is two (at least 2) fold, first, don't let anyone
see object information that they have no 'need to know' clearance for. Second if they really, really,
really must muck with your stuff, at least insulate that with the usual get/set pair (publish through
a controlled interface.) More to the point, globals violate the hell out of these ideas—hence the
bad reputation! On a real world basis, you can obviously do whatever you want, but it is usually better
practice to avoid global scope, only using them when there is a clear need for same. My usual excuse
comes from code optimization, stuffing a variable on the stack when I need to call a function, takes time
so if time is crucial I make the needed stuff global. I'd point out here that if speed is a concern, you
most likely wouldn't be using Perl anyway, more like assembler or C!

To harp on something here ... 99% of the time optimizations are done, it's usually a case of premature optimization.

The basic rule of thumb is that if you cannot back up your desire to optimize with hard data, then DON'T! You will make your code completely unmanageable, usually for only a few micro-seconds of gain - completely unnoticeable to the user.

Clearly your mileage may vary, but the difference between inner loops written in 'C' or higher
compared to assembler (and optimized assembler at that) is way significant if you are talking
about number crunching. Typically I sin in such a fashion when I'm generating fractals and such
like. Being greedy, I will use any trick I can in order to bitblit a little faster!